For a number of electronic devices, high-aspect-ratio semiconducting regions, e.g diodes and isolation grids, extending into or through the thickness of a semiconductor body are desirable.
Direct diffusion of an impurity through the thickness of a semiconductor body from the top surface to the bottom surface through solid material requires diffusion times that are longer than desirable from a productivity standpoint. The semiconducting regions produced by direct diffusion through the solid semiconductor material generally spread further laterally into the interior of the body, away from a path straight through the body, than is desired. The junctions produced between the diffused regions and the body by direct diffusion are generally irregular, e.g. hour-glass shaped, when viewed in the cross-section of the body when the impurity has been diffused from both top and bottom surfaces. These undesirable characteristics of directly diffused through-thickness semiconducting regions become more pronounced as the thickness of the body is increased to provide added strength as the surface dimensions are increased to provide more area for active device fabrication.
Methods are available for enhancing the production of through-thickness regions in semiconductor bodies. One method, such as is described in U.S Pat. No. 4,227,942, involves the use of a chemical etch to expose areas interior to and beneath the surface of the body. These etchants are generally slow acting, produce holes whose dimensions are sensitive to thickness variations unless special procedures are followed, and are sensitive to crystallographic orientations within the body, i.e., the etchants are anisotropic. Since the etchants are anisotropic, the holes produced are typically in the form of truncated tetrahedrons. Hourglass-shaped openings through a semiconductor wafer having semiconducting regions of opposite type lining each opening are shown in "Isolated Power Feed-Thru Holes" by R. C. Joy and W. J. Nestork (IBM Technical Disclosure Bulletin, Vol 16, No. 11, Apr. 1974).
Other methods, such as the one described in U.S. Pat. No. 4,137,100, involve the formation of excavations, i.e., pits or trenches, in the substrate to allow the diffusion processes to commence from levels below the surface of the body. If a laser beam is used to form the excavations, either before or after the impurity is applied, the diffusion time and lateral spreading are decreased, but at the expense of damage, in the form of dislocations or diffusion pipes, to the semiconductor body.
Two additional methods of producing through-thickness diodes are thermal gradient zone melting (TGZM) and electromigration. The advantages of these methods are that they are rapid and produce high quality junctions. The basic nature of the TGZM and electromigration processes, however, limits the choice of impurities which may be used to establish the desired conductivity type and resistivity level of the semiconductor regions relative to that which can be obtained by the more conventional diffusion technology. Further, as the size of the semiconductor regions decreases below about 10 mils, the practice of TGZM and electromigration become increasingly difficult and complex.